A xenobiotic (Greek, xenos "foreign"; bios "life") is a compound that is foreign to a living organism. Principal xenobiotics include: drugs, carcinogens and various compounds that have been introduced into the environment by artificial means.

A substance that does not act as agonist or antagonist but does affect the gamma-aminobutyric acid receptor-ionophore complex. GABA-A receptors appear to have at least three allosteric sites at which modulators act: a site at which benzodiazepines act by increasing the opening frequency of gamma-aminobutyric acid-activated chloride channels; a site at which barbiturates act to prolong the duration of channel opening; and a site at which some steroids may act.

A substance that does not act as agonist or antagonist but does affect the gamma-aminobutyric acid receptor-ionophore complex. GABA-A receptors appear to have at least three allosteric sites at which modulators act: a site at which benzodiazepines act by increasing the opening frequency of gamma-aminobutyric acid-activated chloride channels; a site at which barbiturates act to prolong the duration of channel opening; and a site at which some steroids may act.